1. Field of the Invention:
This invention relates to a method for identifying and tracking special nuclear material (SNM). More particularly it relates to using the emissions from radioactive material to identify and track such material in manufacturing or storage settings.
2. Description of Prior Art:
The identity and location of certain components of manufactured items containing radioactive material has posed a continuing problem. Such material must be closely monitored throughout its movement for the safety of personnel nearby, for national security and for the prevention of theft. Components of complex nuclear devices contain varying amounts of radioactive material known in the industry as special nuclear material (SNM). Some components contain sufficient radioactive material to be dangerous to approach or handle, thus requiring special remote or shielded handling equipment. In addition, such components can have a high intrinsic value running into the thousands of dollars. Other components may be uniquely complex being in high demand by other nations seeking a capability in nuclear technology. For all these reasons, a manufacturer using these materials is held accountable for each and every component, subassembly or finished product. Of economic necessity, he must seek the highest level of security that can be achieved within the limits of the environment involved.
In the prior art, both direct and indirect methods of tracking and identifying radioactive material are found. But in these cases radioactive emitters are purposely placed on items to be tracked, such as vehicles or aircraft. As early as 1961, Cohen et al. (U.S. Pat. No. 3,004,258) discloses the use of radioactive emitters and detectors to control and guide vehicles. Also, the reference suggests a ground based blind landing system using a similar technique. Sarbacher (U.S. Pat. No. 3,099,834) in 1963 discloses the use of radioactive emitters and detectors in a system for aircraft traffic control with the information fed by communication links to a central control station. Chope (U.S. Pat. No. 3,291,987) in 1966 discloses a missile tracking system based on a radioactive source affixed to a missile. Campanella (3,403,254) in 1968 uses penetration radiation for a guide path approach system.
Wright et al., (U.S. Pat. No. 3,508,055) in 1970 discloses perhaps the first use of penetration radiation in the detection of intruders. He relies on the absorption of some of the radiation by an intruder to identify his presence. Koechner (U.S. Pat. No. 4,598,202), later in 1986, discloses the combination of nuclear radiation and light signals from a pressure sensitive optical fiber for a perimeter detection system.
Apparatus and a method for remotely determining the location of unknown nuclear radiation sources is disclosed in 1979 by Rubin (U.S. Pat. No. 4,172,226). He used omnidirectional detectors arranged in a known geometry to feed signals provided by the detectors to a signal acquisition and computer system. A complicated set of equations is used to analyze the ratios of field intensities from the detectors to determine the location of the nuclear source.
In the safety area, Mastain et al. (U.S. Pat. No. 4,480,311) in 1984 discloses a dosimeter with a binary counter for monitoring the extent of radiation to which personnel have been exposed. A portable radiation monitor of the walk-through variety is disclosed by Kruse (U.S. Pat. No. 4,509,042) in 1985. He uses a combination of a counter, software and a microprocessor to sound an alarm when radiation with certain pulse shape is detected. A tamper-indicating seal system is disclosed by Fiarman (U.S. Pat. No. 4,523,186) in a 1985 reference. A break in an optical signal is used to detect unauthorized attempts to obtain access to valuable material such as nuclear material.
Gamma emitters and detectors have also been used in the detection of counterfeit articles such as credit cards. Novik et al. in 1988 discloses a method and apparatus for such detection where the article contains an isotope and the ratio of the gamma rays before and after the article is inserted in the apparatus is used to authenticate the article. Although not directly relevant, conventional radio transmitters and receivers have been used to track patients in nursing homes or similar institutions. Active transceiver and one or more passive transceivers are found in systems (Limis) for controlling manufacturing systems having work stations. Their non-communication detectors include detectors for ions, elemental particles, concentrations, etc. Applications disclosed by Anders et al. (U.S. Pat. No. 4,827,395), in 1989, provide security for valuable items in a home or museum which can be individually coded for identification. Infant security systems using transmitting devices generating coded signals and magnetic strips are disclosed by Wolk et al. (U.S. Pat. No. 4,853,692).
As can be seen, prior art methods and apparatus have contributed much to the breadth of security applications but very few have been addressed to the special problems associated with the monitoring of special nuclear material. Tradeoffs are required between sensitivity of detection and safety. Also, most earlier systems are subject to compromise and deception. The present invention overcomes these drawbacks by utilizing fixed gamma ray detection systems which split the spectrum of the detected gamma ray, the ratios of the upper and lower portions of the spectrum being used to locate and identify positively the nuclear source and any tempering therewith.
Prior art known to this inventor includes the following U.S. Pat. Nos.:
______________________________________ 3,004,258 10/1961 Cohen et al. 3,009,834 7/1963 Sarbacher 3,291,987 12/1966 Chope 3,403,254 9/1968 Campanella 3,508,055 4/1970 Wright et al. 4,172,226 10/1979 Rubin 4,480,311 10/1984 Mastain et al. 4,509,042 4/1985 Kruse 4,523,186 6/1985 Fiarman 4,598,202 7/1986 Koechner 4,742,340 5,1988 Novik et al. 4,814,751 3/1989 Hawkins et al. 4,827,395 5/1989 Anders et al. 4,853,692 8/1989 Wolk et al. ______________________________________